http://courses.washington.edu/bioen508/
For questions, remarks, discussions, errors in the book: Class Discussion Board (link from class website)
Monitored by instructors frequently
Organizer: Paul Kinahan, PhD Adam Alessio, PhD Ruth Schmitz, PhD
Lawrence MacDonald, PhD
Homework for Oct. 25
1. Read Suetens sections 8.5 – 8.10 2. Find 2 medical images of abnormal physiology
using SPECT or PET Place these images in a document • Write 1–2 brief sentences describing each image • Write 1–2 brief sentences describing differences between the
images. • Write 1–2 brief sentences describing what the image values
represent physically.
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Announcements
1. Today’s homework can be handed in tomorrow for those having trouble finding planar gamma camera images.
2. Signup sheet indicating names of groups for class project (Prof. Kinahan will assign undesignated students tomorrow)
3. Field trip to UW Radiology Dept. will be Sat. 10/28 late morning or early afternoon preference?
4. NO CLASS on Nov. 1! The exam will be administered over the web – take-home exam.
Radiation Physics Nuclear Medicine
macdon@u.washington.edu
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• Emission vs. Transmission Imaging
• Nature of nuclear radiation
- Isotopes used in nuclear medicine • Detection methods • Counting statistics • Imaging systems
- Planar gamma scintigraphy
Tomographic systems (SPECT & PET) covered in a later lecture
Emission vs. Transmission imaging
Film or digital x-
ray detector
N0 N
N = N0e-µx —-> N0 known, measure N, infer µ what ambiguity exists?
Transmission imaging; X-ray methods (radiograph, CT, mammography, angiography)
Radiation position & strength (number, energy*) is known
*x-ray source is poly-energetic; averaging required
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detector
Emission imaging Radiation position & strength unknown
Energy is known (mono-energetic)
collimator
Scintillation Camera
Energy Position
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Emission imaging (gamma-ray methods; planar, SPECT, PET) Measures concentration of injected radio- pharmaceutical —-> corresponds to WHAT?
100s of radio-pharmaceuticals designed to highlight a variety of PHYSIOLOGICAL processes. Referred to as
“functional imaging” “molecular imaging”
• Emission vs. Transmission Imaging
• Detection methods • Counting statistics • Imaging systems
- Planar gamma scintigraphy
Nuclear radiation results from unstable nuclei
Nuclear stability is a balance between electromagnetic repulsion of protons and strong force interaction among all nucleons (protons and neutrons).
There are ~ 2,450 isotopes of the ~ 100 elements in the Periodic Table, ~300 of which are naturally occurring, the others are human-made.
Several mechanisms for unstable nuclei to decay to stable isotopes: fission, α-, β-, γ-emission, e- capture.
Frequently there are multiple decay steps to reach stability.
Each decay step is described by an exponential process with a characteristic decay time —-> Half-life of the isotope T1/2 = τ / ln(2)
N=N0e-t/τ
Particle Symbol Mass (MeV/c2) Charge (e-)
Electron e-, β - 0.511 -1
Positron e+, β+ 0.511 +1
Alpha α 3700 +2
Photon γ no rest mass none
Electron mass = 9.11 x 10-31 kg Electron charge = 1.6 x 10-19 Coulombs Joules-to-electronVolts: 1.6 x 10-19 J/eV E = mc2
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• Loses energy in a more or less continuous slowing down process as it travels through matter.
• The distance it travels (range) depend only upon its initial energy and its average energy loss rate in the medium.
• The range for an α particle emitted in tissue is on the order of µm’s.
α Particle Range in Matter 2 protons + 2 neutrons (helium nucleus)
mono-energetic NOT USED FOR IMAGING
- - - - - - - - - - - - - - - - - - - + + + + + + + + + + + + + + + + +
α
• β particle ranges vary from one electron to the next, even for βs of the same energy in the same material.
• This is due to different types of scattering events the β encounters (i.e., scattering events, bremsstrahlung-producing collisions, etc.).
• The β range is often given as the maximum distance the most energetic β can travel in the medium.
• The range for β particles emitted in tissue is on the order of mm’s.
β Decay continuous energy spectrum
Autoradiography (in-vitro imaging) Proximity imaging (? research probes)
β±
Z
Photoelectric effect (µPE) all photon energy transferred to an e-
photon is absorbed; ceases to exist
Compton scattering (µCS) photon ‘bounces’ off an e-
part of the photon energy transferred to the e-, lower energy photon redirected between 0o – 180o
Pair production positron-electron pair is created requires photons above 1.022 MeV
Coherent (Rayleigh) scattering photon deflected with very little energy loss only significant at low photon energies (<50 keV)
λ
Nuclear Medicine Radionuclide Requirements Emission imaging
Charged, massive particles (α-, β-rays) cannot penetrate tissue for emission imaging. —-> need Gamma-Ray emitters [exception: β+ emitters for PET] Half-life
“Too long” leaves damaging radiation in patient after imaging is complete, delivering unnecessary dose. “Too short” does not permit production, preparation, delivery, administration, and internal distribution for practical imaging tasks.
—-> Typically many minutes – hours – a few days is considered about right. Energy of Gamma-Ray
If the energy is “too low” a majority of the photons will be attenuated and not reach the camera (cf. α-, β-rays) . If the energy is “too high” then the γ-rays will pass through the camera without being absorbed by the detector and it is difficult to collimate.
—-> Energies of ~100–500 keV are used. Complexity
A decay scheme with “too many” emissions confounds the imaging process. —-> Select isotopes with relatively simple decays schemes; ideally one or two γ-rays, no β-
or α-rays. Chemical properties
Isotope must be incorporated into a pharmaceutical or other organic compound. —-> Isotopes amenable to chemical, pharmaceutical, and sterile processing.
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List of Nuclear Medicine Radionuclides for “single photon” imaging (i.e. excluding PET)
Isotope Gamma energy(keV) Half-life •• Tc99mTc99m 140.5140.5 6.03 hours6.03 hours • I-131 364, 637 8.06 days • I-123 159 13.0 hours • I-125 35 60.2 days • In-111 172, 247 2.81 days • Tl-201 ~70, 167 3.044 days • Ga-67 93, 185, 300 3.25 days
From: Physics in Nuclear Medicine (Sorenson and Phelps)http://en.wikipedia.org/wiki/Radiopharmaceutical
Technetium-99m
Name Investigation Route of administration In-vitro / in-vivo Imaging / non-imaging Tc99m-pertechnetate Thyroid uptake and thyroid imaging
Stomach and salivary gland imaging Meckel's diverticulum imaging Brain imaging IV In-vivo Imaging Micturating cystogram First pass blood flow imaging First pass peripheral vascular imaging
Tc99m-pertechnetate Lacrimal imaging Eye drops In-vivo Imaging Tc99m-Human albumin Cardiac blood pool imaging IV In-vivo Imaging Tc99m-Human albumin Peripheral vascular imaging IV In-vivo Imaging Tc99m-Human albumin macroaggregates or microspheres Lung perfusion imaging IV In-vivo Imaging Tc99m-Human albumin macroaggregates or microspheres Lung perfusion imaging with venography IV In-vivo Imaging Tc99m-Phosphonates and phosphates Bone imaging IV In-vivo Imaging Tc99m-Phosphonates and phosphates Myocardial imaging IV In-vivo Imaging Tc99m-DTPA (diethylenetriaminepenta-acetic acid) Renal imaging
First pass blood flow studies Brain imaging IV In-vivo Imaging
Tc99m-DTPA (diethylenetriaminepenta-acetic acid) Lung ventilation imaging Aerosol inhalation In-vivo Imaging Tc99m-DMSA(V) (dimercaptosuccinic acid) Tumour imaging IV In-vivo Imaging Tc99m-DMSA(III) (dimercaptosuccinic acid) Renal imaging IV In-vivo Imaging Tc99m-Colloid Bone marrow imaging
GI Bleeding IV In-vivo Imaging Tc99m-Colloid Lymph node imaging Interstitial In-vivo Imaging Tc99m-Colloid Oesophageal transit and reflux imaging Oral In-vivo Imaging Tc99m-Colloid Lacrimal imaging Eye drops In-vivo Imaging Tc99m-HIDA (Hepatic iminodiacetic acid) Functional biliary system imaging IV In-vivo Imaging Tc99m-Denatured red bood cells Red cell volume IV In-vitro Non-imaging Tc99m-Red blood cells GI bleeding
Cardiac blood pool imaging Peripheral vascular imaging IV In-vivo Imaging
Tc99m-MAG3 (mercaptoacetyltriglycine) Renal imaging First pass blood flow imaging IV In-vivo Imaging
Tc99m-HMPAO (Hexamethyl-propylene amine oxime) Cerebral blood flow imaging IV In-vivo Imaging Tc99m-HMPAO (Hexamethyl-propylene amine oxime) labelled leucocytes Infection/inflammation imaging IV In-vivo Imaging Tc99m-Sestamibi Parathyroid imaging
Non-specific tumour imaging IV In-vivo Imaging Thyroid tumour imaging Breast imaging Myocardial imaging
Tc99m-Sulesomab Infection/inflammation imaging IV In-vivo Imaging (IMMU-MN3 murine Fab'-SH antigranulocyte monoclonal antibody fragments) Tc99m-Technegas Lung ventilation imaging Inhalation In-vivo Imaging Tc99m-Human immunoglobulin Infection/inflammation imaging IV In-vivo Imaging Tc99m-Tetrofosmin Parathyroid imaging
Myocardial imaging IV In-vivo Imaging Tc99m-ECD (ethyl cysteinate dimer) Brain imaging IV In-vivo Imaging
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Single photon isotope production Nuclear Reactor Neutron bombardment of target isotopes leads to fission with useful radio-isotope fragments.
Generator Long-lived parent isotope decays to short-lived daughter radio-isotope for use in the clinic (parent produced in, e.g., a nuclear reactor). Daughter separated from parent chemically in the ‘generator’.
Cyclotron Accelerates charged particles (e-, p, α, 2H) that collide with targets resulting in radio-isotopes. Used to generate PET radio-isotopes.
Overview of today’s lecture
• Emission vs. Transmission Imaging
• Nature of nuclear radiation
- Isotopes used in nuclear medicine • Detection methods • Counting statistics • Imaging systems
- Planar gamma scintigraphy
& condition
Basic Radiation Detector Systems What do you want to know about the radiation? Energy? Position (where did it come from)? How many / how much?
Important properties of radiation detectors (depends on application) Energy resolution Spatial resolution Sensitivity Counting Speed
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• Pulse mode – Detect individual photons – Required for NM imaging applications
• Current mode – Measures average rates of photon flux – Avoids dead-time losses – Typically used in x-ray systems (CT)
Types of Radiation Detectors detection modes / functionality
• Counters – Number of interactions – Pulse mode
• Spectrometers – Number and energy of interactions – Pulse mode
• Dosimeters – Net amount of energy deposited – Current mode
• Imaging Systems – CT = current mode – NM = pulse mode
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• Gas-filled detectors
(i.e. another detector)
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Gas-filled Detectors Can be used for imaging, but low sensitivity (low density)
Ionizing event in air requires about 34 eV
From: Physics in Nuclear Medicine (Sorenson and Phelps)
Gas-filled detectors (operates in three ranges)
Geiger-Muller counters
Proportional counters
From: Radiation Detection and Measurement (Knoll, GF)
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ATOMLAB 200 Dose Calibrator
No amplification No dead-time Signal = liberated charge Settings for different isotopes Calibrations
Ionization chamber region
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• Organic solvent – must dissolve scintillator material and radioactive sample
• Primary scintillator (p-terphenyl and PPO) • Secondary solute (wave-shifter) • Additives (e.g., solubilizers) • Effective for measuring beta particles (e.g., H-3, C-14).
Semiconductor Detectors
• Works on same principle as gas-filled detectors (i.e., production of electron-hole pairs in semiconductor material)
• Only ~3 eV required for ionization (~34 eV, air) • Usually needs to be cooled (thermal noise) • Usually requires very high purity materials or
introduction of “compensating” impurities that donate electrons to fill electron traps caused by other impurities
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temperature
Absorption of radiation lifts electrons from valence to conduction band
Impurities (activators) create energy levels within the band gap permitting visible light scintillations
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Effective Atomic Number 51 75 66 59
Attenuation Coefficient (@ 511 keV, cm-1 ) 0.34 0.955 0.833 0.674
Light Output (photons/Mev) 40K ~8K ~30K ~20K
Decay Time 230 ns 300 ns 12 ns 60 ns 40 ns
Wavelength 410 nm 480 nm 420 nm 430 nm
Index of Refraction 1.85 2.15 1.82 1.85
Hygroscopy yes no no no
Rugged no yes yes no
sensitivity
photo-sensor matching manufacturing / cost
Photomultiplier Tube (PMT)
photo-sensor needed with scintillators